A magnetic field is used in Magnetic Resonance Imaging to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This magnetic field is referred to as the B0 field. During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter or amplifier and an antenna cause perturbations to the local magnetic field and can be used to manipulate the orientation of the nuclear spins relative to the B0 field. Spatial encoding of the magnetic spins may be accomplished by using so called gradient coils, which are used to superimpose a magnetic field gradient upon the B0 magnetic field. RF signals emitted by the nuclear spins are detected by a receiver coil, and these RF signals are used to construct the MRI images.
The magnets used to generate the B0 field typically use superconductive coils. The magnetic field generated by the gradient coils can cause eddy currents within the superconductive coils. These eddy currents can be avoided or reduced by using gradient coils with active shielding. U.S. Pat. No. 4,733,189 discloses an active shield about the gradient-forming components of a magnetic resonance imaging system. The actively shielded gradient magnetic field coil known from JP2008-229360 has a field distribution expanding radially and having three longitudinal regions in which a leakage gradient field expands.